In some industrial applications of conveyors there are a number of special performance requirements in addition to common parameters such as speed, weight, and transport capacity. Such applications can be found in the Semiconductor, Pharmaceutical, Solar Cell, Hard Disk Drive, Flat Panel Display, and other manufacturing industries. For these applications and other similar applications, the conveyors used for inter-tool movement of Work In Process (WIP), require “Particulate Free Cleanliness”, “Vibration Free Transport”, “Very High Density WIP Flow”, and “Asynchronous Movements of Pallets with Soft-Accumulation of WIP” (i.e., without collisions or bumping).
Of the above four requirements, current technology has provided for cleanliness, for asynchronous movement, and for soft-accumulation of WIP, e.g., using precisely guided WIP on rollers, driven by motors coupled to the wheels via magnetic hysteresis. See, e.g., U.S. Pat. Nos. 4,793,262 and 6,047,812. Conventionally, a conveyor transporting mechanism consists of a series of wheels supporting and driving a multiplicity of WIP pallets on each of two parallel sides. A magnetic hysteresis coupling allows the driving wheels under a WIP to disengage from the drive shaft of the motor if the inertia of the WIP does not permit synchronization of WIP pallets with the drive speed during acceleration or deceleration, to avoid the squealing of tires.
Advantageously, magnetic hysteresis coupling reduces rubbing motion between driving wheels and WIP pallets, which could otherwise generate particulates that would adversely impact the clean transport requirement. Furthermore, magnetic hysteresis coupling, in combination with segmentation of the conveyor, provides soft accumulation, i.e., without bumping, of WIP pallets because the WIP pallets are guided by presence-of-WIP sensors that define the boundaries of segments on a conveyor that can be occupied by one and only one WIP pallet.
A fundamental drawback of current technology is that the supporting (idling) and driving wheels generate minute vibrations during transport and, therefore, are not able to meet the “vibration-free” requirement. Several physical factors are the cause. First is the near impossibility of manufacturing a large number of wheels to an absolute same diameter and concentricity. Another factor is the practical impossibility of disposing and positioning the wheels to form a straight line, so that any perfectly-planar WIP pallet riding on it would concurrently touch all of the wheels thereunder.
A further drawback of existing practice is a limitation in achieving very high density WIP flow due to relatively moderate acceleration and deceleration rates of the WIP. High density WIP flow requires a relatively close spacing of pallets that travel at high speed. To achieve this in a collision-free environment and in which pallets may move asynchronously of each other requires relatively high acceleration and deceleration rates in case one pallet, for whatever reason, slows or stops.
The physical cause of this drawback is the limited surface contact between WIP pallet undersides and driving wheels necessary for frictional adhesion. Indeed, the friction coefficient of soft or deformable materials is surface area dependent, while hard or more rigid surfaces is less so. As a result, low settings to initiate early disengagement of the magnetic hysteresis drive or clutch, would be necessary, to prevent the spinning of the driving wheels under the pallet during an acceleration mode in which the rubber tires of the clutch-driven wheels are in direct contact with the underside of the driven pallet.
However, low-torque clutch settings cancel higher acceleration rates of the motor driving the clutch. Consequently, high speed and high density of the pallet flow is not currently achievable. Instead, it is important to be able to start a pallet from a standing still position quickly and to stop the same pallet traveling in a high-speed transport mode just as quickly, to maintain the high density of flow.
The need for asynchronous movement of the pallets also necessitates being able to transport each pallet individually if there is space to move the pallet downstream and/or to stop a pallet independently and without bumping if another downstream pallet is obstructing its way. In short, high speed and high density flow, together, require a firm grip on the pallet during its movements. However, individual driving wheels, with the limited surface contact area with the WIP pallet, currently are not able to deliver this performance.
To address these shortcomings, existing conveyor segments, which are structured and arranged to be slightly larger then a WIP or a WIP pallet, can, instead, be equipped with a dedicated drive belt, riding on top of wheels that are independently driven by the same hysteresis clutch/motor mechanism as before. The high-friction belt, sandwiched between the wheels and the WIP pallets, provides necessary adhesion between the WIP pallet and the driving, return idler, and/or idler wheels, to ensure required high, slip-free acceleration. Furthermore, the belt, which is riding on top of the previously disclosed wheels, reduces vibrations generated by any uneven height differences of sequential wheels.
Disadvantageously, generic belt-driven conveyors are not inherently clean. Hence, merely adding belt drives may impact a particulate-free environment. As a result, maintaining a high degree of cleanliness in a belt-driven environment requires special wheel and belt designs.
Accordingly, it would be desirable to provide a high density, high speed, asynchronous belt-driven conveying system that is particulate-free, vibration-free, and that employs soft accumulation.